1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly to a secondary lithium battery having an excellent stability and reliability even with leaving the charged state at a high temperature.
2. Description of the Related Art
As the portable electronic devices such as cellular phones and notebook computers become miniaturized and high-powered and automobiles without environmental contamination are realized, the demand for high power secondary batteries increases explosively, and one of the most noticed batteries among these is based on nonaqueous fluids, that is the lithium secondary battery.
The lithium secondary battery constitutes a cathode and an anode comprising materials that can intercalate and deintercalate lithium ion reversibly, a nonaqueous electrolyte fluid and additional materials that maintain and separate these appropriately. While the lithium secondary battery due to its light weight and excessively low potential has an excellent characteristics of high voltage and capacity, compared to other alkali batteries or nickel-hydrogen, nickel-cadmium batteries, it also has a disadvantage that it can easily short due to the dendrite deposited.
The internal shorting of the lithium secondary battery in the charged state can lead to the problem of firing or explosion caused by the explosive reaction of the organic electrolyte fluid and the active material of the cathode and anode.
Therefore, attempts to ensure the safety in the excessively charged state have been made by the method disclosed in U.S. Pat. No. 5,709,968 where the shut down of the separator is facilitated by adding aromatic compounds such as difluoroanisole when excessively charged and the method disclosed in U.S. Pat. No. 5,776,627 where a current blocking apparatus is operated using additives such as non-phenyl materials that produce gases by polymerization above the normal operation voltage.
Meanwhile, in the case where carbon material is used as an anode of the lithium secondary battery, the solid electrolyte interface (SEI) of a kind of a passivation layer forms in the electrode plate surface, and these SEI films can cause the following problems induced by the reaction with the electrolyte solution at high temperature.
In the formation process, the lithium ion emitted from the lithium complex oxide of the cathode material migrates to the anode and is intercalated, and at this point, the highly reactive lithium ion react with the anode electrode to form Li2CO3, LiO, LiOH, etc, and these compounds form the SEI films at the electrode plate surface.
Since these SEI films are nonconductors, the recharged lithium ions that are moving toward the anode keep the anode material or other materials from reacting. At the same time, because the SEI films work as an ion tunnel and let only lithium ions pass through, the electrode structure is prevented from collapsing by cointercalation, at the anode, of the organic solvent that solvate the lithium ions, thereby carrying the ions. Consequently, once the SEI films form, the amount of the lithium ion can be maintained reversibly and the lifetime characteristics of the battery can also be improved.
The SEI films are relatively solid under the normal condition, i.e., at the temperature range of xe2x88x9220-60xc2x0 C. and at the voltage of less than 4 V and thus they can sufficiently play the role of stopping the negative reaction between the anode and the electrolyte. However, when stored at high temperature in the fully charged state (for example, after charging 100% at 4.2 V, left at 85xc2x0 C. for four days), there is a problem that the durability of the SEI films decays slowly.
In other words, if stored at high temperature in the fully charged state, the SEI films slowly decay due to the chemical and thermal energies increasing as time goes by and thus the anode electrode plate is exposed. The surface of the latter plate exposed in this way reacts with the surrounding electrolyte, and this negative reaction continuously occurs to generate the gases such as CO, CO2, CH4, C2H6, etc., thereby causing an increase in the internal pressure of the battery (J. Power Sources 72, 1998, 66-70).
If the internal pressure of the battery increases, in the case of cylindrical batteries the battery completely loses its function as a battery since it is stopped fully by the operation of the current interrupt device (CID) of excessive current and it reaches a complete stop. Also, in the case of rectangular batteries and pouch type batteries without the CID, a problem arises that it is impossible to load in the main body due to increase in the thickness of the battery occurs.
Meanwhile, Japanese Patent Publication No. hei 10-64591 indicated that as one of the aggravation factors of the cycle characteristics, the organic solvent of the electrolyte disintegrates by oxidation and the disintegration products accumulates on the cathode, thereby impeding preferable reactions in the battery inside, and discloses a method of adding a variety of reduction materials in order to prevent these disintegration by oxidation from occurring. In other words, it means that if the potential of the cathode increases slowly by charging, there can be small spots whose potential is excessively high and strongly oxidative chemical species form in these spots of excessive voltage, and that the disintegration by oxidation of the organic solvent thus occurs. It is also mentioned that in this case, if a suitable additive with a preferable potential (suitable reductivity) is present, this additive first disintegrates by oxidation, thus blocking the disintegration by oxidation of the organic solvent.
Also, Japanese Patent Publication No. hei 10-74537 mentioned that the disintegration of the electrolyte is prohibited by adding an additive that can first react with active oxygen produced in the cathode.
As described above, the prior art mainly use methods that prohibit the disintegration by oxidation of the organic electrolyte by adding an additive that first reacts with the strongly oxidative chemical species produced in the reaction of the electrode and the organic electrolyte.
However, the problems of stability and reliability when left at high temperature remain to be solved.
The technical task the present invention intends to solve is to provide a concept different from the method of the prior art, e.g., a lithium secondary battery that can offer an excellent stability and reliability even when stored at high temperature in the fully charged state.
The technical object of the present invention can be achieved by a lithium secondary battery comprising a nonaqueous organic electrolyte fluid that comprises a cathode electrode containing a lithium complex oxide, lithium metal or its alloy, an anode electrode containing carbon material, and a nonaqueous organic solvent, a lithium salt and an aromatic ether that can react to form dimers or polymers above a certain temperature and voltage, expressed by Formula 1 below. 
wherein, R1 is independently a single bond or an alkylene group with less than or equal to 2 carbons and R2 is hydrogen or an alkyl group with less than or equal to 2 carbons.
According to the examples of the invention, the aromatic ether expressed by the above Formula 1 is preferably diphenyl ether or dibenzyl ether, and its content is preferably 0.1-5.0% by weight with respect to the total weight of the above nonaqueous organic solvent and the lithium salt.
In the present invention, for the lithium complex oxide of active electrode material, the metal lithium of active anode material, its alloy or carbon material, it does not matter that any materials in normal use in this field are used.
Also, as for the nonaqueous organic solvent, it is not particularly limited as long as it can be used in the field of the present invention, and in particular, it is preferably at least one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), ethylmethyl carbonate, methyl acetate, xcex3-butyrolactone, 1,3-dioxolane, dimethoxy ethane, dimethyl carbonate, diethyl carbonate, tetrahydrofuran (THF), dimethyl sulfoxide and polyethylene glycol dimethyl ether.
Also, as for the lithium salt, it is not particularly limited as long as it dissociates to produce the lithium ion, and the particular examples are lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bistrifluoromethanesulfonylamide (LiN(CF3SO2)2), and its content is of the normal level.
In the present invention, the lithium secondary battery may comprise a separator made of multi-porous films.